Ain't No MPDG
The term "manic pixie dream girl" has really taken off since it was coined by Nathan Rabin, a film critic who was responding to Kirsten Dunst's character in Elizabethtown. Here's how Rabin describes such ladies: "[T]hat bubbly, shallow cinematic creature that exists solely in the fevered imaginations of sensitive writer-directors to teach broodingly soulful young men to embrace life and its infinite mysteries and adventures" (source).
Wow, does that not sound exactly like what Joel thinks of Clementine Kruczynski? She's fun and spontaneous, and he is just stuck in a rut. His life is dull and gray, but Clem is going to teach him to live and feel, and it's going to be great. Of course, these dream girls themselves don't really matter: they're always perfectly content, and they're totally on board with fixing the downtrodden male protagonist.
This is really where the whole conflict of the movie starts, even if we don't quite catch on until the end. In Barnes & Noble, for example—only the second time Joel and Clem have seen each other—Clem tells Joel straight up, "Too many guys think I'm a concept, or I complete them, or I'm gonna make them alive. But I'm just a f***ed-up girl who's looking for my own peace of mind. Don't assign me yours."
Joel's response? "I still thought you were gonna save my life, even after that."
Clem is highly aware of how she is viewed She knows that men see her as more of an idea than as a real person; she's an add-on to their lives that relieves them of their own pain or monotony or whatever they're trying to escape. Joel is so blinded by this idea that he practically doesn't hear her.
And it turns out Clem is right. She may be fun, but she's not without her own personality. She's an actual person with actual character, and she definitely has some problems of her own. She might like to drink a bit too much, for one thing, and maybe isn't aware of why Joel would think she's too immature to be a mother.
But the biggest problem is that she gets bored with Joel, and she starts to feel trapped. At first, opposites attract, but over time, that attraction wears off, and both Joel and Clem are repelled by their differences. "Nice" might have sounded great to Clem at first, but in the end, "nice," all by itself, definitely doesn't cut it.
But this is where Clem as a character gets a bit tricky. Most of Eternal Sunshine actually takes place in Joel's mind, meaning that all characters except for Joel himself are merely projections of Joel's subconscious.
In other words, the Clem we see in Joel's head isn't actually Clem—it's merely Joel's projection of Clem, and so we never really know for sure whether this truly represents her character. If all we see of Clem is Joel's projection of her, is it possible for us to know what she is actually like?
Of course, all of Joel's memories are based on something, so even if Clem gets a bit distorted in his mind, there is some basis in reality. In the memories, for example, there's no evidence that Clem sleeps with people to get them to like her, even though Joel accuses her of this on his tape. So it looks like Joel's memories aren't just making stuff up—otherwise, they might show Clem getting around. So if the memories aren't influenced by Joel's anger or frustration in that way, it seems safe to say they are reasonably accurate.
But then who is really coming up with Clem's ideas? Clem, after all, is the one who suggests that Joel try to wake himself up, which almost sort of works. Then Clem has the idea to hide in memories that are off the map, and later on, she suggests they use Joel's repressed memories for extra protection. So if Clem is part of Joel's mind, how is she thinking these things, and not Joel himself?
There's no right answer, but we think Clem might be taking on the savior role that Joel initially gave to her. Joel is stuck thinking of Clem as the manic pixie dream girl he wants her to be, and it's only in Joel's happy memories that Clem actually takes on this role and comes up with great ideas. Maybe Joel's decision to try and hang on to these memories is really a regression back into a flawed understanding of who Clem is and what she can do for him.
That's possible. But the fact that Joel is technically in charge here may show us that he's able to take on this role for himself and be an equal partner for Clem this time around.
It all depends on your perspective.